Introduction:
Sand and stone aggregates are the general term for materials such as sand, pebbles (gravel), crushed stone, block stone, and stone materials in water conservancy and construction projects, and are the main building materials for structures like concrete and masonry. This article summarizes 14 common questions about concrete sand and stone aggregates, hoping to provide assistance to construction projects.
1. What should be noted when coarse and fine aggregates enter the site?
• Coarse aggregates: Mainly control particle size, gradation, particle shape, stone powder content, and mud content. Conduct macro inspections for each vehicle; unqualified materials shall not be unloaded. In addition, inspect all indicators in batches in accordance with specifications.
• Fine aggregates: Control fineness modulus, mud content, and mud lump content. Conduct macro inspections for each vehicle; unqualified materials shall not be unloaded. Similarly, conduct batch inspections in accordance with specifications.
2. Why should the particle size of coarse aggregates be controlled between 5–25mm?
The particle size of coarse aggregates is restricted by the diameter of concrete pump pipes and pumping height. Generally, the ratio of the maximum deliverable particle size to pumping height increases. For example, when the pumping height is <50m, the ratio of maximum coarse aggregate particle size to conveying pipe diameter should be ≤1:3; when the pumping height is 100m, this ratio drops to 1:5, otherwise pipe blockage will occur.
3. Why should the flaky particle content of coarse aggregates be controlled for pumped concrete?
When the flaky particle content is high, the flexural and compressive strength of flaky coarse aggregates is relatively low, and the bonding strength between coarse aggregates decreases, thereby reducing the concrete strength. For ready-mixed concrete, a high flaky particle content will result in poor particle shape of coarse aggregates, reducing the fluidity of concrete. At the same time, flaky coarse aggregates can easily block pipes, causing pump blockage or even pipe bursts. Therefore, pumped concrete requires a flaky particle content ≤10%, and high-strength concrete has higher requirements.
4. What kind of sand is needed for ready-mixed concrete?
Ready-mixed concrete requires medium sand. In addition to meeting specifications for sand gradation, mud content, and mud lump content, attention should also be paid to ensuring that the content passing through a 0.315mm sieve is no less than 15%. This has a significant impact on the pumpability of concrete; if this value is too low, pipe blockage will occur, and the concrete will have poor water retention and be prone to bleeding.
5. What impact does too-fine sand have?
If the sand is too fine, the water demand of concrete increases, and the concrete prepared with fine sand has poor workability and water retention, leading to reduced concrete strength and easy cracking.
6. What if only fine sand is available?
If there is a problem with the sand source, pumped concrete can be prepared by mixing fine sand with some manufactured sand. For example, fine sand with a fineness modulus less than 2.0 can be mixed with manufactured sand with a fineness modulus of 3.0–3.2 at a ratio of about 6:4. Observe its fluidity and pumpability, and specific adjustments can be made through trial mixing.
7. What are the consequences of excessive sand mud content?
Excessive mud content in sand increases the water demand of concrete, reduces water retention, increases shrinkage, reduces concrete strength, and makes structures prone to cracking. Therefore, the mud content should be controlled to ≤3% (for C30–C50), and high-strength concrete has higher requirements.
8. How does mud lumps in sand and stone affect concrete?
In addition to the same adverse effects as mud, mud lumps in sand and stone can severely reduce concrete strength. For example, mud lumps can weaken the fracture surface of concrete; when pouring the ground, mud lumps float up and form concave defects on the surface after drying and shrinkage.
9. Why should smaller-sized stones be used when preparing high-strength concrete?
• As the particle size of coarse aggregates increases, the bonding with cement paste weakens, increasing the discontinuity of the internal structure of concrete materials and leading to reduced concrete strength.
• Coarse aggregates constrain the shrinkage of cement in concrete. Due to the different elastic moduli of coarse aggregates and cement paste, tensile stress is generated inside the concrete. This tensile stress increases with the increase of coarse aggregate particle size, further reducing concrete strength.
• As the particle size of coarse aggregates increases, the orientation of Ca(OH)₂ crystals at the interface transition zone of coarse aggregates increases, weakening the interface structure and reducing concrete strength.
Tests show:
• In concrete with 15–25mm coarse aggregates, the interface crack width around the aggregates is about 0.1mm, the crack length is 2/3 of the particle circumference, and interface cracks are more connected to cracks in the surrounding cement paste.
• In concrete with 5–10mm coarse aggregates, the interface crack width is uniform, only 0.03mm, and the crack length is only 1/6 of the particle circumference.
• For coarse aggregates of different sizes, the water pockets formed under the particles after concrete hardening accumulate differently. Larger coarse aggregates have larger and more water pockets. After the water in the water pockets evaporates, the interface cracks formed are wider than those of smaller particles, resulting in lower interface strength.
10. Why is the strength of pebble concrete 3–4MPa lower than that of crushed stone concrete with the same mix ratio?
The rough surface of coarse aggregates is conducive to the interface strength between cement paste and aggregates. According to years of tests, concrete prepared with pebbles, on the one hand, contains more weathered stones, and its own crushing index is lower than that of crushed stone. On the other hand, its smooth surface leads to low interface strength, so the strength of concrete prepared with it is 3–4MPa lower than that of crushed stone concrete with the same mix ratio.
11. Why should the moisture content of sand and stone be measured every shift?
Sand and stone account for about 800–1100kg/m³ in ready-mixed concrete. Every 1% moisture content will affect the water consumption in concrete by 8–11kg. Sand, usually collected from rivers, has a large change in moisture content. If the moisture content is not detected regularly and the mixing water volume is adjusted in a timely manner, it will cause large fluctuations in the slump, workability, and strength of each batch of concrete.
12. What is alkali-aggregate reaction?
Alkali-aggregate reaction refers to the chemical reaction between alkalis in concrete and aggregates with active silica as the chemical composition, generating alkali-silica gel that absorbs water and swells. The expansion stress causes concrete cracking.
13. How to prevent alkali-aggregate reaction?
If the local coarse aggregates contain active silica, the alkali content of concrete admixtures must be strictly limited. According to the "Code for Design of Concrete Structures" (GB50010-2002), when using alkali-active aggregates, the total alkali content of all materials in concrete should be ≤3% of the concrete mass.
14. What items should be inspected when cement enters the site?
For every 500t of cement, random sampling should be conducted for inspections such as compressive strength, flexural strength (3d, 28d), consistency water consumption, initial and final setting time, and soundness.
